The present invention relates to a phase correction circuit for a transistor using a high-frequency signal, which is used for a radio communication device, for example.
It has been well known to those skilled in the art that a threshold voltage VTH of a gate of a transistor is increased by an increase in the temperature of the transistor. If the threshold voltage VTH has been increased, a high-level voltage of a control signal supplied to the gate needs to increase up to a value corresponding to the increased threshold voltage VTH, so that the transistor can work correctly. Otherwise, other circuits provided on the downstream side of the transistor and using an output signal of the transistor cannot work correctly.
In order to solve the problem, a temperature compensation circuit has been proposed so far. The circuit supplies a compensation voltage which increases together with the increase in temperature of the transistor, so that the transistor can work correctly, even if the high-level voltage of the control signal is fixed irrelevant to its temperature change.
FIG. 14 shows a depletion type n-channel field effect transistor 100 of which a gate is connected to not only a control signal line but also a temperature compensation circuit 10. A threshold voltage VTH of the transistor 100 is xe2x88x921.5V. A drain of the transistor 100 is supplied with a voltage Vcc. A source of the transistor 100 is connected to the ground. Also, the drain of the transistor 100 is connected to a high-frequency circuit 200.
The temperature compensation circuit 10 functions as a potential dividing circuit. The circuit 10 has a load circuit 11 and a resistor 12 connected in series to the load circuit 11. The joint P1 between the load circuit 11 and the resistor 12 is connected to the gate of the transistor 100. More particularly, the load circuit 11 has three diodes 11a, 11b and 11c, connected in series and supplied with a forward bias. A terminal 13 beside the load circuit 11 is supplied with Vg1=xe2x88x921v. A terminal 14 beside the resistor 12 is supplied with Vg2=xe2x88x925v. A resistance Rd of the load circuit 11 is increased by a temperature increase. This is caused by a well-known temperature characteristic of the diode being supplied with forward bias, i.e., each resistance of the forward biased diode 11a, 11b and 11c is increased by the temperature increase.
A potential of the joint P1, or a compensation voltage being supplied to the gate of the transistor 100 is increased by an increase in resistance Rd of the load circuit 11 with the temperature increase. An incremental ratio of the compensation voltage for the temperature increase is determined identically to that of the threshold VTH of the transistor 100 for the temperature increase. Therefore, the transistor 100 can work correctly, even if the high-level voltage control signal is not changed with the increase in temperature of the transistor 100.
As described above, the temperature compensation circuit 10 can supply the compensation voltage increasing together with the increase in temperature of the transistor 100 to the gate of the transistor 100. If the transistor 100 having the temperature compensation circuit 10 is used as a transistor for a high-frequency circuit, it causes a problem that a phase of output signal from the transistor 100 is shifted together with the increase in supply voltage, because a depletion capacitance increases together with the increase in supply voltage. If an amount of phase shift is increased, a following high-frequency circuit 200 can not work correctly.
As the phase correction circuit for a transistor using a high-frequency signal, several circuits have been proposed in various documents: a phase temperature compensation high frequency amplifier in JP A 03-258008, a semiconductor device and amplifier in JP A 11-74367, and a peaking circuit in JP A 01-137713, for example.
The phase temperature compensation high frequency amplifier in JP A 03-258008 has a circuit functioning as a phase correction circuit. The circuit has a varactor diode and a potential dividing circuit, adjusting a supply voltage of the varactor diode. The potential dividing circuit uses a positive thermistor having a resistance varying with its temperature. It is well known by those skilled in the art that the relationship of the capacitance of the varactor diode to the supply voltage is determined by a state of a p-n junction. That is, at the step junction, the depletion capacitance of the varactor diode changes in simple proportion to the square root of the supply voltage. In addition, at the graded junction, the depletion capacitance of the varactor diode changes in inverse proportion to the cube root of the supply voltage. On the contrary, the depletion capacitance of the transistor changes in simple proportion to the increase of its temperature. Therefore a compensated phase does not indicate any constant value in relation to its temperature change, or cannot have a linear relationship as described in JP A 03-258008. In addition, the semiconductor chip size becomes large because the amplifier needs to include not only the varactor diode but also a potential dividing circuit.
The semiconductor device and amplifier in JP A 11-74367 uses a diode that is inverse connected to another diode in an equivalent circuit of a transistor. The circuit supplies an inverse bias to the inverse connected diode. An electric potential of the inverse bias is set the same potential of the diode in the equivalent circuit. Therefore the capacitance changes of each of the diodes are canceled and therefore secondary unsymmetrical wave distortion can be removed. Nevertheless, this circuit cannot correct a phase shift caused by the increase in threshold voltage. In addition, JP A 11-74367 fails to disclose any circuit to correct a phase shift being caused by the temperature increase.
The peaking circuit in JP A 01-137713 can adjust a phase of an output signal from a transistor by means of a side-gate at a constant environment temperature. However, the circuit is not constructed to correct a phase shift caused by the increase in threshold voltage VTH due to the temperature increase. Also, in this third document, there is no description of a circuit to correct a phase shift caused by the temperature increase. In addition, the compensation circuit described in the third document is an only circuit to cancellation an influence from the side-gate.
Therefore, a purpose of the present invention is to provide a phase correction circuit to stabilize a phase of an output signal of the transistor, even if its gate potential is increased by a temperature compensation function, the temperature increase and the other reasons.
To this end, the phase correction circuit for the transistor using high-frequency signal, comprising: a) a circuit element connected in parallel to a gate of the transistor together with a control signal line, an impedance including a reactance component of the circuit element being changed by a potential difference between an input terminal and an output terminal of the circuit element; and b) a voltage control circuit for adjusting a supply voltage to the circuit element to decrease the reactance component in response to an increase in potential of the gate, wherein a total value of reactance components of the circuit element and the transistor is set to a predetermined value, so that another circuit using an output of the transistor can work correctly.
In another aspect of the present invention, the circuit element may be a diode of which a cathode is connected to the gate of the transistor. In this case, the voltage control circuit supplies predetermined reverse bias to the diode.
In another aspect of the present invention, the circuit element may be a diode of which a cathode is connected directly or indirectly to the gate of the transistor and a transmission line connected in series to the anode or cathode of the diode. In this case, the voltage control circuit supplies predetermined reverse bias to the diode.
In another aspect of the present invention, the circuit element may be two diodes of that the cathodes connected each other. One anode of the two diodes is connected to the gate of the transistor. In this case, the voltage control circuit supplies predetermined reverse bias to another anode of the two diodes.